1. Technical Field
The present invention relates to a method of manufacturing an optical fiber preform using a plasma torch such as a radio-frequency induction thermal plasma torch.
2. Description of the Related Art
A plasma torch such as a radio-frequency induction thermal plasma torch is structured by arranging a radio-frequency coil around a tube providing a gas channel and operated through application of a radio-frequency current to the coil to convert the gas in the tube into plasmas and to eject the plasmas. Such a plasma torch can achieve an ultrahigh temperature of approximately 10,000 degrees, provides a relatively low plasma linear speed, and allows the oxidizing/reducing atmosphere to be freely selected. Thus, such a plasma torch is used to provide an ultrahigh-temperature reaction medium.
An optical fiber that is structured such that a pure silica glass core is covered with a fluorine-doped silica glass cladding is characteristically highly ultraviolet- and radiation-resistant, when compared with a normally-used optical fiber that is structured such that a germanium-doped silica glass core is covered with a pure silica glass cladding. This is because the former has no Ge—O bonds, which have a low bonding energy.
There are some known methods for forming a fluorine-doped silica glass cladding on a glass core. A first method is disclosed, for example, in Examined Japanese Patent Publication No. 04-079981 and includes forming a porous glass layer around a pure silica glass rod by depositing pure silica glass fine particles and vitrifying the porous glass layer in a fluorine-containing atmosphere into a transparent glass. A second method is disclosed, for example, in Examined Japanese Patent Publication No. 02-047414 and includes directly depositing a transparent fluorine-doped silica glass around a pure silica glass rod with the use of a plasma flame.
The first method can only be used to achieve a relative refractive index difference up to approximately 0.7%, but exhibits excellent productivity and is suitable for forming a thick cladding layer. The second method is less productive than the first method, but can accomplish a relative refractive index difference higher than 0.7%.
The second method is now described with reference to FIG. 1.
A plasma torch 1 has a coil 2. When a radio-frequency power is applied to the coil 2, the gases supplied from a gas supply device 3 are converted into plasmas in the plasma torch 1 and the plasmas are ejected in a plasma flame 4. The gases include, for example, argon, oxygen, silicon tetrachloride, and a fluorine-containing gas (silicon tetrafluoride, hexafluoroethane, sulfur hexafluoride or the like).
In the plasma flame 4, fluorine-doped glass fine particles are produced and deposited onto the surface of a glass rod 6, which is rotated and reciprocated in the upward and downward directions in a reaction chamber 5. The glass fine particles that do not attach to the glass rod 6 and the waste gas are discharged outside through a gas outlet 7. In this manner, fluorine-doped glass thin-films are repeatedly deposited until an optical fiber preform having a cladding layer with a predetermined thickness is manufactured.
In the optical fiber preform manufactured in the above-described method, the relative refractive index difference disadvantageously varies in its longitudinal direction. This variation results from the varying refractive index of the cladding layer that is formed by the plasma depositing step since the refractive index of the core remains constant over the entire length. Furthermore, the optical fiber preform manufactured in the above-described method only achieves a low relative refractive index difference. This is because the temperature of the glass rod is not constant in its longitudinal direction during the depositing step.
To solve the above-mentioned problems, Japanese Patent Application Publication No. 2008-134585 discloses a method according to which the depositing step is carried out while the glass rod is moved in a first direction (for example, in the downward direction) but suspended while the glass rod is moved in a second direction opposite to the first direction (for example, in the upward direction). While the glass rod is moved in the second direction, the plasma torch is supplied with a lower power and the glass rod is moved faster than while the glass rod is moved in the first direction, for the purpose of suppressing the rise in the temperature of the glass rod. According to this method, the temperature of the glass rod remains substantially constant in the longitudinal direction during the depositing step. Consequently, the relative refractive index difference remains constant over the entire length of the resulting optical fiber preform.
According to the method disclosed in Japanese Patent Application No. 2008-134585 in which the deposition of the glass fine particles is carried out while the glass rod is moved in the first direction or downwards and the deposition is suspended while the glass rod is moved in the second direction or upwards, the deposited glass layer does not have a smooth surface in a lower portion of the glass rod (a portion having a predetermined length from the end of the glass rod at which the deposition starts) and may have a rough surface. The length of the rough-surface portion varies depending on the shape of the end of the glass rod, and may range from approximately 20 to 60 mm with respect to the lower end of the glass rod (the end of the glass rod at which the deposition starts), for example, when the glass rod has a diameter of 50 mmφ. Such a rough-surface portion corresponding to the lower portion of the glass rod cannot be used as an optical fiber base material. Accordingly, the above-mentioned method has a problem of a low yield.
If a higher power is supplied to the plasma torch to reduce the roughness, the fluorine doping amount decreases in the straight portion, which results in a lower relative refractive index difference and thus in poorer optical characteristics. This is because, as disclosed in Japanese Patent Application No. 2008-134585, the temperature of the glass rod is interrelated with the fluorine doping amount, specifically speaking, as the temperature of the glass rod increases, the fluorine doping amount decreases.